<!DOCTYPE html>
<html>
    <head>
        <style>
            html,body{
                margin: 0;
                width: 100%;
                height: 100%;
                display: flex;
            }
            .left{
                width: 400px;
                height: 100%;
                background-color: #cccccc;
            }
            .right{
                flex: 1;
                height: 100%;
                background-color: red;
            }
            
        </style>
    </head>
    <body>
        <div class="left">left</div>
        <div class="right">right</div>
    </body>
</html>